In cellular wireless systems, base stations often need to communicate user data/information to multiple wireless terminals simultaneously. In the downlink, the link from the base station (BS) to the wireless terminals (WTs), an important problem is the allocation of base station transmitter power to the different WTs being served simultaneously by the same BS. Each BS typically has a total transmit power budget available for all downlink communication, and this power is typically shared amongst the WTs. The base station transmitter power allocated to a WT in the cell will influence the WT's received signal-to-noise ratio (SNR), which in turn affects the downlink data rate that the wireless communications link from the base station to the WT can support. In this way, the power allocation can be used to adjust the downlink data rate to different WTs depending on their traffic needs and channel conditions.
One wireless system where this power allocation problem arises is a multiple user orthogonal frequency division multiplexing (OFDM) wireless communications system supporting concurrent transmission of different downlink user data to different wireless terminals sourced from the same base station transmitter.
One problem in realizing the potential of the multiple access OFDM downlink, is that a base station needs to perform appropriate power allocation. For any assigned data rate option of a downlink traffic segment, the BS needs to correctly allocate enough transmitter power for that segment to be received reliably at the intended WT. If too little power is allocated, the decoding of the segment will likely fail and need re-transmission. If the power allocated for is excessive, it means that power was wasted and that wasted power could have been used for the other WTs being serviced by the base station.
Ideally, each data rate option that can be used for downlink traffic communication has a corresponding minimum received SNR requirement, and ideally the received SNR will scale linearly with the received power. Consequently, under ideal circumstances, the WT could measure the SNR at a single reference signal level, and then report that SNR back to the BS. Knowing that the SNR scales linearly with the power, assuming an ideal case, for any scheduled data rate option, the base station could adjust the transmit power relative to the reference signal to insure that the segment is received with the correct SNR for that data rate.
However, in practice, the WT receiver processing introduces errors, such as channel estimation inaccuracies, phase jitter, and timing and frequency offsets. These errors typically scale with the received power, and effectively add a signal-dependent component to the noise. This noise component is sometimes called “self-noise,” to distinguish it from external and thermal noise that is independent of the signal processing. In the presence of self-noise, the received SNR no longer scales linearly with the received power. In particular, as the received power is increased, the SNR eventually saturates at a maximum level depending on the self-noise.
In the presence of self-noise, the WT can no longer simply report the SNR at a single power level and expect the base station to be able to determine correct transmit power corresponding to different data rate options. From a single SNR measurement, the BS cannot separate the self-noise and external noise components, and therefore, cannot accurately extrapolate the power required to obtain any other SNR.
The problem of self-noise is particularly important in recently developed wireless technologies which offer high downlink data rates. These systems offer rates at high SNRs (often in excess of 20 dB) where the self-noise component can be significant. Also, as these services are to be offered in mobile, fading environments, or in long range applications with significant delay spread, the self-noise component will become more pronounced. It is thus important that the BS can properly select its transmit power corresponding to different downlink traffic channel segments to account for self-noise.
Consequently, there is a need in wireless communications systems for methods and apparatus directed to the measurement, determination, reporting, and/or use of wireless terminal self-noise information.